Historic changes in sea level can indicate how it will change in future. Most of our sea-level records are based on the chemical make-up of sediment cores, which are hard to date ‐ estimates can be thousands of years out. As a result, it is unclear how soon sea level started rising after Earth warmed in prehistoric times, how quickly it rose and what we can expect in the future.

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Rohling’s team dated the stalactites by measuring how much of the rock’s uranium has decayed into thorium. Because the chemical makeup of the stalactites changed over time in the same way as that of the sediment cores, the two can be directly compared. By comparing the stalactites and sediments, the group can estimate when sea level changes occurred with a resolution of about 200 years – the most accurate yet.

Rohling’s study suggests that the most alarming predictions of sea-level rise can be ruled out.

Slower but for longer

Earlier studies had suggested that we could expect sea level to rise by several metres per century, but Rohling reckons that 1m per century is about the maximum. His record shows a similar rate of rise during the last interglacial period 129,000 years ago, when sea levels were close to today’s.

Rohling’s prediction aligns with estimates made by the majority of climatologists today, including Jerry Mitrovica of Harvard University, who was not involved in the study. “Those [high] rates of ice volume reduction were taking place when there was a lot more ice around,” he says. Big ice sheets lose mass faster than smaller ones, producing faster sea level rises.

But even if future sea level rise is quite slow, the cumulative effect will be large as the phenomenon will continue for centuries. “We’ve put the freight train in motion and it’s going to be very difficult to slow it down,” Rohling says.